MU-MIMO (Multiple User-Multiple Input Multiple Output) system is important means to increase peak data rate and spectral efficiency in current communication in which one main application is closed-loop downlink pre-coding. In such an application, when signals are sent to user equipment by multiple antennae located in a base station, the user equipment measures a downlink transmission channel from the base station to the user equipment, and then suggests a corresponding pre-coding matrix (index) for the base station based on the channel condition. The suggestion is transferred to the base station via a feedback link. Subsequently, during the downlink transmission, it is possible for the base station to directly adopt the suggested pre-coding matrix to pre-code the signal to be sent to the user equipment.
To facilitate MU-MIMO operation, advanced feedback schemes are being considered in 4G wireless standards, such as LTE-A. One of those advanced feedback schemes is BCI (Best Companion PMI (Pre-coding Matrix Index)).
In completed LTE Rel-8, the feedback scheme of PMI can well support for SU-MIMO (Single User-Multiple Input Multiple Output) system. By such feedback scheme, a UE (User equipment) reports the most preferred pre-coding matrix to be used for the UE itself to the base station. In the subsequent LTE standards such as LTE Rel-10, in addition to PMI, the UE also reports to the base station the most preferred pre-coding matrix to be used for other UEs, i.e. BCI, if the other UEs are co-scheduled with the UE.
FIG. 1 is a diagram showing a system in which a mobile UE reports PMI and BCI to a base station. In the MU-MIMO system as shown in the FIG. 1, when the base station 100 transmits signals to the UE 102 through antennae located in the base station 100, the UE 102 measures downlink transmission channel from the base station 100 to the UE 102, and then suggests a corresponding PMI for the base station 100 based on the channel condition. In addition to the PMI used for the UE 102 itself, the LIE 102 also reports to the base station 100 the most preferred BCI to be used for other UEs, such as the UE 104, co-scheduled with the UE 102.
FIG. 2 is a diagram showing the signaling for reporting BCI from the UE to the base station. As shown in the FIG. 2, similar to the feedback scheme of PMI and CQI (Channel Quality Indicator) in LTE Rel-8, in addition to BCI, the UE 102 also reports to the base station 100 ΔCQI (delta CQI) which indicates the CQI degradation due to the co-scheduling of the other UEs. In the FIG. 2, “t” denotes time and “T” denotes transmission cycle (period) of signaling, wherein the corresponding BCI and ΔCQI is transmitted from the UE 102 to the base station 100 every a certain cycle T.
According to some practical applications, BCI and ΔCQI may be fed back to the base station within the same signaling as or different signaling from that for feeding back the PMI and/or CQI.
FIG. 3 is a diagram showing a system in which a UE reports a PMI and multiple BCIs to a base station. In the MU-MIMO system as shown in the FIG. 3, the base station 300 respectively transmits the signals to multiple UEs 302, 304 and 306 through antennae located in the base station 300, and the UE 302 measures downlink transmission channel from the base station 300 to the UE 302, and suggests a corresponding PMI for the base station 300 based on the channel condition. In addition to the PMI used for the UE 302 itself, the UE 302 also reports to the base station 300 the most preferred BCIs, i.e. BCI1 and BCI2, to be used for the other two UEs 304 and 306 co-scheduled with the UE 302, respectively. Reporting multiple BCIs can greatly increase scheduling flexibility at base station side, thus improving MU-MIMO performance.
FIG. 4 is a diagram showing the signaling for reporting multiple BCIs to the base station from the UE. As shown in the FIG. 4, the UE 302 feeds back BCI1 and BCI2 to the base station 300 at the timings t1, t2, t3 and t4 respectively. In addition to BCI1 and BCI2, the UE 302 also reports the corresponding ΔCQI1 and ΔCQI2 to the base station 300. However, it can also be seen from the FIG. 4 that reporting multiple BCIs results in feedback overhead distinctly increasing. In the FIG. 4, the number of corresponding BCIs and ΔCQIs transmitted from the UE 302 to the base station 300 at a certain cycle T is two times of that of the FIG. 2.